We have developed a computational procedure for determining high resolution structures of large icosahedral virus particles. The thrust of the procedure is to determine accurately the centers and orientations of particles in focal-pairs of 400 kV, spot-scan micrographs by iterative common-lines-based local and global refinement. We choose shared-memory multiprocessor computers for executing the global refinement, which is the most computationally-intensive step in the reconstruction procedure. The speedup and efficiency of our parallel codes are evaluated on two different computer architectures. We show that parallel computing permits the reconstruction of large icosahedral viruses at higher resolutions than was previously practical and has allowed us to refine the 9 [unreadable] structure of herpesvirus B-capsid from hundreds of noisy, close-to-focus images. The map shows the sub-domain features of the protein subunits, which exhibit differences at quasi-equivalent positions and reveals large variations in the numbers and locations of contact points between penton/hexon subunits and their adjacent triplexes. This extensive conformational switching of proteins represents departures from the pattern predicted by the Caspar-Klug theory and presumably reflects structural compromises made to keep this large capsid stable.